Nitrogen is separated from a nitrogen and oxygen containing feed, typically air, within a cryogenic plant employing at least one distillation column. Such plants are known in the art as nitrogen generators. The compressed and purified feed is cooled to a temperature suitable for rectification within the distillation column by way of a main heat exchanger. The resultant cooled feed is then introduced into a bottom region of the distillation column to produce a nitrogen-rich vapor as a column overhead and an oxygen-rich liquid column bottoms.
In order to reflux the column, a heat exchanger is employed to condense a stream of the nitrogen-rich vapor by indirect heat exchange with a stream of the oxygen-rich liquid column bottoms that is depressurized and cooled by way of an expansion valve. A further part of the nitrogen-rich vapor can be warmed within the main heat exchanger against cooling incoming feed along with an oxygen enriched waste stream that is produced by substantially vaporizing oxygen-rich liquid column bottoms. The oxygen enriched waste stream may be warmed directly in the main heat exchanger or partially warmed and turboexpanded to produce an exhaust stream that may then be fully warmed within the main heat exchanger. Waste turboexpansion serves to cool the exhaust stream in order to refrigerate the plant. Alternatively, the expansion of part of the incoming feed or product nitrogen streams can be used for such purposes of refrigeration.
Typically, the heat exchanger used to generate reflux for the distillation column functions by way of a thermosiphon effect. In a thermosiphon driven heat exchanger, the depressurized oxygen-rich liquid column bottoms is introduced into a vessel containing the heat exchanger. The heat exchanger is at least partially submerged within the liquid. Liquid entering the heat exchanger passages is partially vaporized which decreases the stream density and makes the liquid rise within the heat exchanger. During the process, the liquid indirectly exchanges heat with the condensing nitrogen-rich vapor. There exist several problems with a thermosiphon type heat exchanger. In general, the exit vapor fraction required to sustain adequate circulation is quite low. As a consequence, substantial mixing losses are incurred (the oxygen concentration of the boiling pool is substantially higher than the concentration of the depressurized column bottoms). In addition, a larger temperature difference must be maintained in order to overcome the static head created by the submergence of the heat exchanger. These factors translates into higher plant compression requirements relative to plants employing down-flow heat exchangers which allow closer temperature approaches with reduced levels of recirculation.
In the prior art, down-flow heat exchangers have been described extensively for use in high purity oxygen plants. The down-flow heat exchanger is disposed within the bottom region of the lower pressure column to evaporate an oxygen-rich liquid column bottoms against the condensing nitrogen-rich column overhead of a higher pressure column. Down-flow heat exchangers can incorporate a plurality of parallel plates and fins to form passages for the fluids between the plates. Additionally, down-flow heat exchangers can have a shell and tube configuration in which the nitrogen-rich vapor is fed to the shell that contains tubes supported by opposed tube sheets and the oxygen-rich liquid is distributed to the tubes by way of a liquid distributor. As the liquid falls within such a heat exchanger, the oxygen-rich liquid partly vaporizes to condense the nitrogen-rich vapor.
In practically employing down-flow heat exchangers in the prior art, the oxygen-rich liquid from the sump of the lower pressure column is recirculated to the liquid distributor to prevent all of the liquid from being evaporated. In one patent, U.S. Pat. No. 5,799,510, this circulation is provided by an ejector in which liquid oxygen contained in the sump of the low pressure column is pumped to produce a high pressure oxygen product. Part of the pump stream is introduced as a motive fluid into the ejector for such recirculation purposes. Alternatively, in U.S. Pat. No. 5,924,308, a valve is used to partially depressurize a lower pressure column liquid oxygen stream. The two phase mixture is recondensed in a separate heat exchanger and fed back to the top of the down-flow heat exchanger. This patent discloses a process which is complicated by additional heat exchanger equipment and spatial requirements. Furthermore its application is directed at high purity oxygen plants.
Neither of these two methods is particularly advantageous for a single column nitrogen generator because generally there is no pressurized product that can serve as a motive fluid and the use of a separate heat exchanger will negatively impact fabrication costs.
As will be discussed, the present invention provides a cryogenic rectification plant employing a distillation column for producing a nitrogen-rich vapor as column overhead in which reflux is generated for the column with the use of a down-flow heat exchanger in which circulation is produced by an ejector but the motive fluid is advantageously the oxygen-rich liquid column bottoms rather than the residual sump liquid produced by the down-flow heat exchanger.